Conventionally, a composite reverse osmosis membrane, which is obtained by forming an active skin layer substantially having selective separating properties on a microporous support, has been known as a reverse osmosis membrane whose structure is different from that of an assymmetric reverse osmosis membrane.
There have been composite reverse osmosis membranes in which a skin layer comprising polyamide obtained by interfacial polycondensation of polyfunctional aromatic amine and polyfunctional aromatic acid halide is formed on a support (for example, Japanese Laid-Open Patent Publication Nos. 55-147106, 62-121603 and 63-218208, Japanese Patent Publication No. 6-73617, and the like).
The composite reverse osmosis membranes described above have a high desalination performance and a high flux performance. However, it has been desirable that the composite reverse osmosis membranes should further improve the flux performance while keeping a high desalination performance in respect of a reduction in operation and equipment costs, efficiency and the like. For these requirements, various additives have been proposed (for example, Japanese Laid-Open Patent Publication No. 63-12310), which have improved performance but are insufficient. Furthermore, after-treatment of the composite reverse osmosis membrane performed by using chlorine and the like has also been proposed (for example, Japanese Laid-Open Patent Publication Nos. 63-54905 and 1-180208).
However, the reverse osmosis membrane according to the prior art has a salt rejection of 98.9% and a flux of 2.2 m.sup.3 /m.sup.2.multidot.day at an operating pressure 15 kgf/cm.sup.2 as an example of the highest flux. When operation is performed at a pressure of 7.5 kgf/cm.sup.2, the flux is reduced to about half and the salt rejection is considerably lowered. For this reason, the reverse osmosis membrane could not be practically used at a pressure of 7.5 kgf/cm.sup.2. Moreover, the flux was rarely obtained at a pressure of about 3 kgf/cm.sup.2. Therefore, the reverse osmosis membrane could not be used. Accordingly, a composite reverse osmosis membrane having a high desalination performance at a low pressure has been required with respect to economy.